The ability to measure, nondestructively and in situ the thickness of growing thin films is very advantageous in many industrial applications. For example, of paramount importance to passenger safety is the ability to monitor in situ growth of ice coatings on aircraft. Buildup of ice layers on aircraft wings or other materials has been and continues to be a cause of aircraft disasters. During or after takeoff of the aircraft the added weight of the ice, which can be very significant, as well as the accompanying change in aerodynamic flow patterns over the airflow surfaces can cause crashes. Preventative procedures such as de-icing the aircraft typically are carried out when the aircraft is near the hangar after which the aircraft taxis to the end of the runway for takeoff. During this period ice can again build up on the aircraft depending on the distance the aircraft must taxi and the severity of the weather conditions.
One current method of measuring ice thickness on an airfoil uses microwave electromagnetic radiation. The microwave radiation is used to monitor the thickness and dielectric constant of the growing layer from which the composition is calculated. U.S. Pat. Nos. Pat. Nos. 4,054,255 and 4,688,185 issued to Magenheim and Magenheim et al. respectively disclose using a dielectric layer affixed to the wing surface as a surface waveguide into which a low power microwave signal is directed. The impedance and reflection properties of the waveguide change as ice builds up on the waveguide and this change is measured and related to the buildup of ice.
Drawbacks to microwave monitoring systems are the expense of the power supplies and the need for sophisticated software for handling the data. Microwave monitoring systems necessitate cutting holes in the airfoils of the aircraft or otherwise modifying the structure to include waveguide elements which increase installation costs, disturb the flow pattern over the airfoil and may reduce structural strength.
Another known method for monitoring ice build-up involves the use of internal reflection to measure ice thickness. U.S. Pat. No. 4,797,660 issued to Rein Jr. teaches use of internal reflection of EM using a prism mounted to the wing surface. A light source and detector are positioned to cause light to impinge on the exposed surface of the prism and a detector measures internally reflected light from the exposed surface with the reflected intensity being a function of the buildup on the exposed prism surface. U.S. Pat. No. 5,296,853 issued to Federow et al. is directed to a laser ice detector comprising a light source, light detector and temperature sensor with the light source and detector embedded in a plastic housing mounted flush with the surface of the wing. The system is designed to give total internal reflection when ice is absent from the plastic surface. The presence of ice on the plastic is accompanied by loss of total internal reflection.
U.S. Pat. No. 4,797,660 issued to Michoud et al. discloses an ice thickness measuring technique for aircraft using internal reflection of light. The device is designed to discriminate against water and ice with for example falling rain acting to modulate the light signal received by the detector in a characteristic manner thereby distinguishing it from the signal due to ice buildup. As with microwave techniques, a drawback to internal reflection is the need for modification of the airfoil surface.
Patent No. GB 1385279 discloses a device for detecting ice on the surface of an aircraft including a radioactive source producing gamma rays or beta rays (fast electrons) and a pair of Geiger-Muller detectors located laterally of the radiation source with one detector on each side of the source. This device has several disadvantages. The radiation detector must operate under very cold conditions and since Geiger-Muller counters are known to be very inefficient detectors of 60 keV gamma rays, the radiation sources must have a very high strength. Further, there is nothing in the design of the ice measuring device which makes the system more sensitive to scattering in an ice layer as opposed to scattering in the air-foil material.
A rapid and accurate method of measuring the build-up of ice on aircraft in flight is required for safety considerations. Ice build-up occurs predominantly on the ground and at low altitudes with little build-up occurring at normally high cruising altitudes for jet aircraft. However, the steady increase in air traffic unaccompanied by construction of more airports has resulted in the practice of "stacking up" low priority flights before giving clearance to land. This is particularly the case in inclement weather and at relatively low altitudes, conditions most conducive to icing. Therefore, there is a need for a rapid, accurate, economic, in situ and nondestructive method of measuring the thickness of growing films on substrates.